Add fail-safe to "Arrays" category methods to allow falsy values for `array` argument

As issue #23 points out, sometimes it's nice to have array methods not error when they receive a falsy value for the required `array` argument.

The use case is something like

``` js
// should handle when `String#match` returns `null` without erroring
_.flatten(string.match(/regexp/))
```

So I would like to know if you all (/cc @kitcambridge @mathiasbynens) think it's worthwhile.
